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><A
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>7.3. Using a swap space</A
></H1
><P
>An initialised swap space is taken into use with
	<B
CLASS="COMMAND"
>swapon</B
>.  This command tells the kernel that
	the swap space can be used.  The path to the swap space is given
	as the argument, so to start swapping on a temporary swap file
	one might use the following command.

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><PRE
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><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>swapon /extra-swap</B
></TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
></FONT
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>

	Swap spaces can be used automatically by listing them in
	the <TT
CLASS="FILENAME"
>/etc/fstab</TT
> file.

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>/dev/hda8        none        swap        sw     0     0
/swapfile        none        swap        sw     0     0</PRE
></FONT
></TD
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>

	The startup scripts will run the command <B
CLASS="COMMAND"
>swapon
	-a</B
>, which will start swapping on all the swap
	spaces listed in <B
CLASS="COMMAND"
>/etc/fstab</B
>.  Therefore,
	the <B
CLASS="COMMAND"
>swapon</B
> command is usually used only when
	extra swap is needed.</P
><P
>You can monitor the use of swap spaces with
	<B
CLASS="COMMAND"
>free</B
>.  It will tell the total amount of swap
	space used.

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><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>free</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>             total       used       free     shared   
 buffers
Mem:         15152      14896        256      12404       2528
-/+ buffers:            12368       2784
Swap:        32452       6684      25768</TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
></FONT
></TD
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>

	The first line of output (<TT
CLASS="LITERAL"
>Mem:</TT
>) shows the
	physical memory.  The total column does not show the physical
	memory used by the kernel, which is usually about a megabyte.
	The used column shows the amount of memory used (the second
	line does not count buffers).  The free column shows completely
	unused memory.	The shared column shows the amount of memory
	shared by several processes; the more, the merrier.  The buffers
	column shows the current size of the disk buffer cache.</P
><P
>That last line (<TT
CLASS="LITERAL"
>Swap:</TT
>) shows similar
	information for the swap spaces.  If this line is all zeroes,
	your swap space is not activated.</P
><P
>The same information is available via
	<B
CLASS="COMMAND"
>top</B
>, or using the proc filesystem in file
	<TT
CLASS="FILENAME"
>/proc/meminfo</TT
>.  It is currently difficult
	to get information on the use of a specific swap space.</P
><P
>A swap space can be removed from use with
	<B
CLASS="COMMAND"
>swapoff</B
>.  It is usually not necessary to do it,
	except for temporary swap spaces.  Any pages in use in the swap
	space are swapped in first; if there is not sufficient physical
	memory to hold them, they will then be swapped out (to some other
	swap space).  If there is not enough virtual memory to hold all
	of the pages Linux will start to thrash; after a long while it
	should recover, but meanwhile the system is unusable.  You should
	check (e.g., with <B
CLASS="COMMAND"
>free</B
>) that there is enough
	free memory before removing a swap space from use.</P
><P
>All the swap spaces that are used automatically
	with <B
CLASS="COMMAND"
>swapon -a</B
> can be removed from use
	with <B
CLASS="COMMAND"
>swapoff -a</B
>; it looks at the file
	<TT
CLASS="FILENAME"
>/etc/fstab</TT
> to find what to remove.
	Any manually used swap spaces will remain in use.</P
><P
>Sometimes a lot of swap space can be in use even though
	there is a lot of free physical memory.  This can happen for
	instance if at one point there is need to swap, but later a big
	process that occupied much of the physical memory terminates
	and frees the memory.  The swapped-out data is not automatically
	swapped in until it is needed, so the physical memory may remain
	free for a long time.  There is no need to worry about this,
	but it can be comforting to know what is happening.  </P
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